1. Field of the Invention
The present invention relates to a magnetoresistive random access memory (MRAM) and a manufacturing method thereof, and more particularly to an MRAM and a manufacturing method thereof which can simplify the whole process and increase a memory speed by improving an MRAM cell structure.
2. Description of the Background Art
Most of the semiconductor memory manufacturing companies have developed an MRAM using a ferromagnetic material as one of the next generation memory devices.
The MRAM is a memory device for reading and writing data by forming multi-layer ferromagnetic thin films, and sensing current variations according to magnetization directions of the respective thin films. The MRAM has a high speed, low power consumption and high integration density because of the special properties of the magnetic thin films, and performs a nonvolatile memory operation like a flash memory.
The MRAM research is still in its early stage, and mostly concentrated on the formation of multi-layer magnetic thin films, less on a unit cell structure and a peripheral sensing circuit, etc.
FIG. 1 is a cross-sectional diagram illustrating a magnetic tunnel junction (MTJ) cell having a multi-layer magnetic thin film structure for storing data in the conventional MRAM.
In general, the MTJ cell 5 includes a anti-ferroelectric thin film 1, a pinned ferromagnetic thin film 2, a thin insulation layer 3 which tunneling current flows through, and a free ferromagnetic thin film 4.
Here, the magnetization direction of the pinned ferromagnetic thin film 2 is pinned to one direction, and the anti-ferroelectric thin film 1 fixes the magnetization direction of the pinned ferromagnetic thin film 2. Conversely, the magnetization direction of the free ferromagnetic thin film 4 is changed by an external magnetic field.
When current flows through the MTJ cell 5 in the vertical direction, tunneling current is generated through the insulation layer 3. When the pinned ferromagnetic thin film 2 and the free ferromagnetic thin film 4 have the same magnetization direction, the tunneling current increases, and when the pinned ferromagnetic thin film 2 and the free ferromagnetic thin film 4 have opposite magnetization directions, the tunneling current decreases.
This phenomenon is called a tunneling magnetoresistance (TMR) effect. The magnetization direction of the free ferromagnetic thin film 4 can be sensed by sensing the amount of the tunneling current, and thus data stored in the cell can be read in the read operation.
In the write operation, the magnetization direction of the free ferromagnetic thin film 4 is decided according to polarity of current by a level of voltage transmitted to a write word line, and thus the MTJ cell 5 stores a data of ‘0’ or ‘1’.
FIG. 2a is an exemplary diagram illustrating an MRAM cell using a horizontal structure field effect transistor (FET).
The unit cell of the MRAM includes one horizontal structure FET 9, an MTJ cell 5, a read word line 6, a write word line 8 and a bit line 7.
The read word line 6 is used to read data to control the FET 9. The write word line 8 receives current, forms an external magnetic field, and changes the magnetization direction of the free ferromagnetic thin film 4 of the MTJ cell 5 by using the external magnetic field, to store data. The bit line 7 transmits current to the MTJ cell 5 in the vertical direction, to sense the magnetization direction of the free ferromagnetic thin film 4.
In the read operation, the conventional MRAM transmits voltage to the read word line 6 to operate the FET 9, transmits current to the bit line 7, and senses the amount of current flowing through the MTJ cell 5.
In the write operation, the MRAM maintains an off state of the FET 9, transmits current to the write word line 8 and the bit line 7, and thus changes the magnetization direction of the free ferromagnetic thin film 4 of the MTJ cell 5 by the external magnetic field.
Here, the current is transmitted to the bit line 7 and the write word line 8 at the same time because the magnetic field is most actively generated in the vertical crossing point of the two metal lines. Accordingly, one cell can be selected from a few cell arrangements,
FIG. 2b is a cross-sectional diagram illustrating an MRAM corresponding to the conventional MRAM cell of FIG. 2a. 
A ground line 12 is formed on a source 10 of the horizontal structure FET 9, the read word line 6 is formed on a gate thereof, and a conductive layer 13, a contact plug 14, a conductive layer 15 and a contact plug 16 are sequentially formed on a drain 11 thereof. A connection layer 17 is formed on the write word line 8, and the MTJ cell 5 and the bit line 7 are stacked on the connection layer 17.
One of the biggest problems in the manufacturing process of the MRAM is to control surface roughness of the lower structure in which one MTJ cell 5 is formed below a few nm. In the MRAM of FIG. 2b, the write word line 8 and the connection layer 17 are formed in the lower portion of the MTJ cell 5, and thus surface roughness is hardly controlled below a few nm.
In addition, the conventional MRAM includes two word lines and one bit line per unit cell as well as a ground line for grounding, namely four metal lines, which increases surface resistances. It is very important to control the resistances.
When the transistor is small, it is much more difficult to control the external resistance of the transistor. Such resistances in combination with the resistances of the MTJ cell have detrimental effects on the cell operation. A short channel effect and resistance variation of the horizontal structure FET deteriorate a few hundreds Giga-level integration of the MRAM.